In a stand mixer bowl, combine flour, baking powder and oil.
In a small bowl, add the warm water, sugar, and yeast. Let the yeast activate and become bubbly.
Once the yeast has activated, with the mixer running on low, add it to the flour mixture. Knead until the dough is smooth and elastic, about 5 minutes. If the dough is too wet, add a little more flour. If the dough is too dry, add 1 teaspoon of water at a time until you obtain the correct consistency.
Lightly grease a medium glass bowl and place the the dough into it. Cover and let it proof in a warm place for about 1 hour or until almost doubled in bulk.
Once dough has risen, divide into portions of ~50g each. Roll each dough portion into a smooth ball and flatten the edges to form a disc with a hump in the middle.
Remove the custard filling balls from the freezer and place one custard ball in the middle of the dough. Carefully bring up the edges of the dough and pleat around it, sealing up the filling.
Place the dough with the pleats facing down onto a square piece of parchment paper, approximately 3" by 3" in size.
Repeat with the remaining dough and filling.
Place the custard buns (with the parchment) onto a steamer rack or a bamboo steamer.
Let rest for about 20-30 minutes, until dough has risen slightly.
Use large pot or wok with just enough water and set your steaming rack over top of it. Turn on the heat to high and steam for 15 minutes.
Turn off the heat and let it rest for 5 minutes without opening the lid. (This is to prevent the buns from collapsing).
Remove from heat and let cool. Enjoy the custard buns immediately.